This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. We recently demonstrated partial efficacy of PC-815, a microbicide gel consisting of the NNRTI MIV-150 and Carraguard (CG) against vaginal challenge of rhesus macaques with SHIV-RT (SIVmac239 expressing HIV-1 reverse transcriptase). Here, we aimed to further improve the potency of the gels by adding zinc, which enhances the anti-HSV-2 activity of CG in mice. We tested two candidate microbicides: PC-707 (14mM zinc acetate in CG) and PC-1005 (50mM MIV-150 and 14mM zinc acetate in CG). Treatment gels PC-707 (n=14) and PC-1005 (n=21), or methyl cellulose (MC) placebo (n=14), were applied daily (2ml/day) for 14 days to the vaginal epithelium of Depo-Provera-treated rhesus macaques. Animals were vaginally challenged with 103 TCID50 SHIV-RT at 4, 8, or 24h after the last gel application. Follow-up included detection of plasma virus RNA and PBMC virus DNA, seroconversion, and IFN-[unreadable] responses. Statistical significance was determined with Fisher's exact test. In the MC group, 12 of 14 animals (85.7%) became infected. Among the PC-707 treatment groups, 1 of 7 (14.3%) and 2 of 7 animals (28.6%), became infected when challenged 8 and 24h after final gel application, respectively. None of the animals treated with PC-1005 became infected with SHIV-RT after challenge at any time point. This contrasts findings from separate studies where we found that in animals treated with CG, 8 (5 of 7 animals or 71.4% infected) or 24h (4 of 7 animals or 57.1% infected) and PC-815 (50 M MIV-150 in CG) in animals challenged 8 (2 of 7 animals or 28.6% infected) or 24h (5 of 7 animals or 71.4% infected) after the last gel application. PC-1005 was significantly more protective than CG (p0.0003). Infection typically correlated with the development of SIV-specific Ab and T cell responses. Repeated daily application of PC-1005, a microbicide containing MIV-150 and zinc acetate delivered in CG, provided complete protection for up to 24h against vaginal SHIV-RT challenge. The enhanced protection mediated by PC-1005 (compared to the partial activity seen when either MIV-150 or zinc acetate alone were delivered in CG) is promising for the development of an effective, coitally-independent gel.